Like others have mentioned its really about stats and IPs want to work with someone who has a much better chance of carrying a healthy baby to term.
But, there is a big difference between being overweight, or having a higher bmi because your muscular and being obese.
Obesity is liked to health concerns for both the baby and the surrogate, and in the long run being obese is strongly associated with all kinds of health issues(cancer, cardiovascular disease, diabetes). Some doctors consider being obese to be just as dangerous for your health as smoking.
If surrogacy is something you want to do then it may just be the inspiration you need to get your BMI down into the healthier range.
Mom to twins born May 2010